1. Field of the Invention
The present invention relates to a high-frequency circuit device, such as an oscillator, using a slot line, and to a communication apparatus having the high-frequency circuit device.
2. Description of the Related Art
For example, an oscillating circuit is available as a high-frequency circuit that is incorporated in a communication apparatus or the like. Attempts for reducing the phase noise have been made to improve the characteristic of the oscillating circuit. It is known that the phase noise can be reduced, for example, by increasing the Q-factor of the oscillating circuit. Accordingly, techniques for reducing the phase noise have been proposed by connecting a resonator to an oscillating circuit to thereby increase the Q-factor of the oscillating circuit (see non-patent document 1, as an example).
Non-patent Document 1: xe2x80x9cProceedings of 1994 IEICE (The Institute of Electronics, Information and Communication Engineers) Fall Conferencexe2x80x9d, Funabashi et al., C-60
In this proposed oscillator (a high-frequency circuit device), for example, an oscillating circuit is provided on a substrate and a resonator, which is in the form of a chip component, is mounted on a surface of the substrate.
With the above proposed configuration, however, the positional accuracy for mounting the resonator on the substrate is poor, which poses a problem in that characteristic variations occur depending on each oscillator.
In addition, another configuration for an oscillator has been proposed (see non-patent document 2, as an example).
Non-patent Document 2: xe2x80x9cProceedings of 1998 Electronics Society Conference of IEICExe2x80x9d, Ikematsu et al., C-2-15
In the document, a signal line in an oscillating circuit is formed of a microstrip line or coplanar line on a semiconductor substrate, and an FET (field effect transistor) element is formed, as an amplifying element, inside the semiconductor substrate. Further, a microstrip resonator or coplanar resonator is formed at the semiconductor substrate so as to provide a monolithic configuration.
In that configuration, the semiconductor substrate is used as the circuit board. To enhance the Q-factor of an oscillating circuit, it is preferable that a circuit board is made of material that is low in dielectric loss tan xcex4. However, there is no semiconductor material that has dielectric loss (tan xcex4) low enough to provide a satisfactory Q-factor. Thus, with the configuration of the proposed oscillator using a semiconductor substrate, an improvement in the Q-factor of an oscillating circuit has been limited.
Accordingly, the present invention has been made to overcome the above problems, and an object of the present invention is to provide an improved high-frequency circuit device whose Q-factor can be further reduced and whose phase noise can be substantially reduced and to provide a communication apparatus having the high-frequency circuit device.
To achieve the foregoing object, a first aspect of the present invention provides a high-frequency circuit device. The high-frequency circuit device includes a substrate and a high-frequency circuit provided on the substrate. The high-frequency circuit has a signal line, and the signal line is configured with a slot line provided by electrodes that are arranged side-by-side with a space therebetween on the substrate.
According to the present invention, a signal line included in a high-frequency circuit is provided by a slot line. As compared to a coplanar line for transmitting a high-frequency signal, a slot line can reduce conduction loss and also can easily prevent the generation of an unwanted wave. Thus, a slot line can enhance the Q-factor of a high-frequency circuit and can reduce the phase noise thereof. As a result, the use of a slot line can provide a high-frequency circuit device that has superior characteristics.
Meanwhile, a microstrip line, which is one type of transmission line for transmitting a high-frequency signal, is likely to have a parasitic component, which makes the circuit design difficult. In contrast, a slot line is less likely to generate a parasitic component, which can ease the difficulty of circuit design. Thus, a slot line is advantageous in terms of circuit design.
In addition, with a coplanar line or a microstrip line, the thickness of the substrate needs to be increased to enhance the Q-factor of a circuit, and thus the increased thickness of the substrate results in a problem of heat dissipation. In contrast, with a slot line, it is easy to enhance the Q-factor without increasing the thickness of the substrate, which can alleviate the problem of heat dissipation.
Preferably, a semiconductor element is provided along the slot line and is a surface-mounted component, which is mounted to the slot line on the substrate by using bumps. Since the slot line is provided using bumps, this arrangement can reduce the disturbance of an electromagnetic field at a portion where the slot line and the semiconductor are connected. This can reduce loss due to the leakage of an unwanted wave.
Additionally, the substrate, on which the high-frequency circuit is provided, is made of dielectric material. Thus, an appropriate selection of a dielectric material for constituting the substrate can substantially reduce the dielectric loss of the substrate and can significantly improve the Q-factor. That is, a decrease in conduction loss due to the use of a slot line and a decrease in dielectric loss due to the use of a dielectric substrate are combined to thereby significantly enhance the Q-factor of a high-frequency circuit. This arrangement, therefore, can easily provide a high-frequency circuit device that is superior in phase noise characteristic.
A slot line through which a high-frequency signal is transmitted may include a wide portion for reducing conduction loss. This arrangement can reduce the conduction loss in the slot line, can enhance the Q-factor of the circuit, and can reduce the phase noise.
The output portion of the high-frequency circuit may include a coplanar line. This arrangement facilitates the connection of a signal line to a subsequent circuit, since a signal line of a subsequent circuit connected to a high-frequency circuit device is configured with a coplanar line in many cases.
Preferably, the high-frequency circuit is an oscillating circuit, and a semiconductor element serving as an amplifying element is provided along the slot line. Configuring a signal line with a slot line can reduce the conduction loss of an oscillator circuit and thus can improve the Q-factor of the circuit. The use of a dielectric substrate for the substrate, on which the oscillating circuit is provided, facilitates a reduction in dielectric loss resulting from the substrate and thus can improve the Q-factor of the circuit. Since this arrangement allows the Q-factor to be enhanced as described above, it can provide an oscillating circuit whose phase noise is reduced.
A field effect transistor may be used for the amplifying element. The slot line is preferably provided by an inter-gate/source slot line, an inter-gate/drain slot line, and an inter-drain/source slot line, which form a feedback circuit. This arrangement provides a simple circuit configuration and can facilitate miniaturization of a high-frequency circuit device (an oscillator). The inter-gate/drain slot line is used to control the amount of feedback. Since the frequency of a feedback signal is controlled, this arrangement can suppress parasitic oscillation in the oscillating circuit and thus can improve the stability of the oscillation frequency of the oscillating circuit.
The gate-drain slot line that provides the feedback-amount controlling portion in the feedback circuit may have a short stub provided at a position of about xcex/4 from a portion where the inter-gate/drain slot line, the inter-gate/source slot line, and the inter-drain/source slot line are interconnected. This arrangement facilitates the control of the amount of feedback and an oscillation frequency.
In addition, the substrate may have a resonator, which is connected to the oscillating circuit. This arrangement can further reduce phase noise in the oscillating circuit and can further stabilize the oscillation frequency of the oscillating circuit.
In addition, configuring the resonator with a conductor pattern can miniaturize the oscillator. Further, since the resonator and the slot line can be formed at the same time, it is possible to prevent a variation in the geometric relationship between the slot line and the resonator and to prevent a characteristic variation resulting therefrom. This arrangement, therefore, can improve the reliability of a product quality.
The resonator may be configured with a TE-mode plate resonator or a slot-line resonator. Since the use of such a resonator can easily improve the Q-factor of the resonator, it is possible to further improve the Q-factor and to achieve a reduction in phase noise.
Since a bipolar transistor has small low-frequency noise compared to a field effect transistor, it is possible to reduce phase noise.
The oscillating circuit may include an external-voltage-control-type variable-capacitance element. This arrangement allows the capacitance of the variable capacitance element to be variably controlled by an external voltage control, so that the capacitance of the variable capacitance element varies the oscillation frequency of the oscillating circuit. This allows for an external control of the oscillation frequency of the oscillating circuit.
When the resonator is provided, preferably, an external-voltage-control-type variable-capacitance element for variably controlling the resonant frequency of the resonator is connected to the resonator. Variably controlling the capacitance of the variable capacitance element with an external voltage can vary the resonant frequency of the resonator, which in turn can vary the oscillation frequency of the oscillating circuit. In this case, it is possible to widen the variable control range of the oscillation frequency of the oscillating circuit compared to a case in which the oscillating circuit includes the variable capacitance element.
Preferably, the substrate is made of dielectric material and has an f xe2x96xa1 Q-factor of 50000 or more. This arrangement facilitates the Q-factor of the high-frequency circuit to be enhanced, and can reduce phase noise, as described above.
Another aspect of the present invention provides a communication apparatus that has the high-frequency circuit device using the slot line. Since the high-frequency circuit device of the present invention has improved performance, this arrangement stabilizes the operation of the communication apparatus and can improve the characteristics of the communication apparatus.